The present invention relates to handling of substrates that undergo a xe2x80x9cthrough-etchxe2x80x9d micromachining process, and more particularly to a method that provides a xe2x80x9chandlexe2x80x9d wafer with projecting support posts that interlock with complementary posts on the bottom side of a wafer to be through-etched (i.e., the product wafer).
In bulk silicon micromachining, a silicon wafer is etched to form mechanical beam elements in microelectromechanical systems (MEMS) devices such as microsensors and microactuators. This etching is often performed by an anisotropic plasma-etching process. In deep-trench reactive-ion etching (RIE), etches are often made through the entire wafer (forming trenches/holes from the front side of the wafer to the back side), defining a high-aspect ratio mechanical structure. However, two primary issues are faced when performing through-wafer etching.
First, a gas such as helium is often impinged on the back side of the wafer being etched to cool it during the etching process. However, as trenches are etched completely through the wafer, holes are created that allow helium to leak from the back side of the wafer to the front, etching side. This helium leakage causes undesirable localized and wafer-level effects in the etching due to variations in gas composition, plasma, and wafer cooling. Further, all such plasma etches have some inherent nonuniformity. Therefore, etching through the wafer results in parts releasing at different times. To etch through all devices on the wafer, a significant overetch (i.e., etching for an amount of time between when the first parts are released and the last parts are released) is required following the initial breakthrough. The primary problem associated with overetching is variations in feature dimensions that result from some areas being etched more than others, which causes marked changes in the dynamics of the different devices.
A second problem encountered with through-etching is when the etched silicon parts xe2x80x9creleasexe2x80x9d from the wafer (i.e., any connections between a device and the wafer are etched away, thus separating the device from the wafer). This separation may be either intentional or unintentional. It is desired to have these released, singulated devices unloaded from the chamber along with the product wafer rather than letting pieces fall directly on the chamber cathode (which supports the product wafer during the through-etch process). In the latter case, the singulated devices are left behind in the chamber after wafer unloading, requiring further processing to retrieve the devices.
A common approach to these problems associated with through-etching is to attach an unpatterned xe2x80x9chandlexe2x80x9d wafer to the non-etch side of a product wafer using photoresist as a temporary adhesion layer. An example of such an approach appears in Field et al. U.S. Pat. No. 5,882,532. However, this presents many drawbacks such as photoresist residue left on the attached silicon surface, causing difficulties in releasing the etched wafer following the through-etch. Moreover, the photoresist in contact with the bottom side of the product wafer also changes the etching characteristics due to photoresist decomposition. This method can sometimes be used in a research environment, but is not a manufacturable solution for through-wafer etching of MEMS devices.
Another approach to through-etching involves silicon-on-insulator (SOI) wafers, in which product wafers are bonded to a handle wafer using an insulating bond layer such as silicon dioxide. The etch process proceeds down to the interlayer oxide, serving as an xe2x80x9cetch-stop.xe2x80x9d Non-insulating films (such as a metal thin film) can also be used. However, both insulating and non-insulating films require a xe2x80x9crelease stepxe2x80x9d to remove the residual layer from the device following etching. This is often extremely undesirable or incompatible with MEMS processes. In addition, when an insulating layer such as silicon dioxide is used as an etch stop in narrow trenches during deep-trench RIE processes, an undesirable xe2x80x9cfooting effect,xe2x80x9d or lateral etching of RIE ions caused by charging in the oxide layer, is typically observed at the oxide/silicon interface.
The present invention is a method of handling a wafer for through-wafer plasma etching. Lateral support is provided between a handle wafer and a product wafer without wafer bonding or an adhesive film (such as photoresist) using mechanically mating structures. The product wafer is easily separated from the handle wafer following etching without stripping or cleaning. Because the connection between the wafers is mechanical, rather than an adhesive/bonded layer, a wafer can be etched, inspected, and subsequently etched again without the hindrance of repeated bonding, separation, and cleaning. A non-bonded support for released devices following a through-etch process is also provided.